docs: def Sun_Duffey_Peng(x, rhol, rhog, sigma, m, D, P, Pc, g=g):
      r'''Calculates void fraction in two-phase flow according to the model of 
      [1]_ as given in [2]_ and [3]_.
      
      .. math::
          \alpha = \frac{x}{\rho_g}\left[C_0\left(\frac{x}{\rho_g} + \frac{1-x}
          {\rho_l}\right) +\frac{v_{gm}}{G} \right]^{-1}
          
      .. math::
          v_{gm} = 1.41\left[\frac{g\sigma(\rho_l-\rho_g)}{\rho_l^2}\right]^{0.25}
          
      .. math::
          C_0 = \left(0.82 + 0.18\frac{P}{P_c}\right)^{-1}
          
      Parameters
      ----------
      x : float
          Quality at the specific tube interval []
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
      sigma : float
          Surface tension of liquid [N/m]
      m : float
          Mass flow rate of both phases, [kg/s]
      D : float
          Diameter of the channel, [m]
      P : float
          Pressure of the fluid, [Pa]
      Pc : float
          Critical pressure of the fluid, [Pa]
      g : float, optional
          Acceleration due to gravity, [m/s^2]
  
      Returns
      -------
      alpha : float
          Void fraction (area of gas / total area of channel), [-]
  
      Notes
      -----
          
      Examples
      --------
      >>> Sun_Duffey_Peng(0.4, 800., 2.5, sigma=0.02, m=1, D=0.3, P=1E5, Pc=7E6)
      0.7696546506515833
  
      References
      ----------
      .. [1] K.H. Sun, R.B. Duffey, C.M. Peng, A thermal-hydraulic analysis of
         core uncover, in: Proceedings of the 19th National Heat Transfer 
         Conference, Experimental and Analytical Modeling of LWR Safety 
         Experiments, 1980, pp. 1-10. Orlando, Florida, USA.
      .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
         Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
         1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
      .. [3] Woldesemayat, Melkamu A., and Afshin J. Ghajar. "Comparison of Void 
         Fraction Correlations for Different Flow Patterns in Horizontal and 
         Upward Inclined Pipes." International Journal of Multiphase Flow 33, 
         no. 4 (April 2007): 347-370. doi:10.1016/j.ijmultiphaseflow.2006.09.004.
      '''
      G = m/(pi/4*D**2)
      C0 = (0.82 + 0.18*P/Pc)**-1
      vgm = 1.41*(g*sigma*(rhol-rhog)/rhol**2)**0.25
      return x/rhog*(C0*(x/rhog + (1-x)/rhol) + vgm/G)**-1